A prior art AC (=alternating current) voltage regulator (also called sliding voltage regulator), commonly known as `slidac`, has, for example, a circuit configuration shown in FIG. 1 or FIGS. 2(a) and 2(b).
The AC voltage regulator shown in FIG. 1 is a typical example of mechanical type. With this mechanical type, it is general that the voltage regulation is carried out manually, but there have also been commercially available those which detect an output voltage with a controller A and maintain a constant voltage automatically by means of a reversible motor (not shown).
However, for example, the prior art AC voltage regulator of mechanical type with manual voltage regulation of 1 KVA has a weight of about 8 kg and the prior art AC voltage regulator of mechanical type with motor-driven automatic voltage regulation of 1 KVA has a weight of as much as 12 kg, thus the prior art regulator is very heavy, and also its response speed with respect to input voltage fluctuation is very slow because of its mechanical control.
In order to increase the response seeed, there has been developed an AC voltage regulator which performs a pulse width control, that is, an On/Off control of AC voltage waveform using a high-speed semiconductor switch.
FIG. 2(a) is a circuit diagram showing an example of an AC voltage regulator of this electronic control type. In the AC voltage regulator of the electronic control type shown in FIG. 2(a), a controller B controls a semiconductor switch C and second semiconductor switch D to be ON and OFF alternately, and with this ON/OFF alternation, a pulse width control of an AC input voltage E.sub.0 is performed, thereby controlling continuously an output voltage E.sub.0 D from around zero voltage to the vicinity of the input voltage E.sub.0 by varying a duty ratio D from substantially zero to 1. In this case, for example, an AC voltage regulator of an 1 KVA output with a weight of about 4 kg has been realized, thus the electronic control type is much lighter than the mechanical control type and also has an improved response speed with respect to input voltage fluctuation.
However, such a prior art AC voltage regulator of electronic control type has problems relating to boosting of its input voltage.
In order to raise the output voltage higher than the input voltage, for example, it is necessary to add a low-frequency step-up transformer D as shown in FIG. 2(b), and as a result, the volume and the weight of the AC voltage regulator increase considerably and the advantages of it being an electronic-control-type are reduced to the sole merit that its response speed with respect to input voltage fluctuation is fast.